1. Field of the Invention
The present invention relates to an optical signal switching unit comprising optical switches employed for optical communication.
2. Description of the Background Art
An optical transmission system is widely employed as a transmission line in communication on the Internet. This optical transmission system is formed by an optical fiber member employed as a transmission line for light and various types of electronic devices including an optical device. The optical transmission system transmits mass data through a single optical fiber member, and hence a fault caused in the transmission line has a tremendous influence. Therefore, the optical transmission system must be highly reliable, and have a spare transmission line for preliminarily performing optical transmission following development of a fault. In general, the main transmission line developing a fault is switched to the spare transmission line with an electric signal, and the spare transmission line is switched to the main transmission line when the same is restored to the normal state.
An optical signal switching unit employing optical switches implements high reliability of the optical transmission system. The optical signal switching unit processes parts, such as switching to the spare transmission line, generally processed with an electric signal, with an optical signal for switching from the spare transmission line to the main transmission line upon restoration to the normal state. When such an optical signal switching unit is introduced, the network of the optical transmission system is so simplified that various types of electronic devices employed for the optical transmission system can be miniaturized, improving economy.
The aforementioned optical transmission system employs a device consisting of an optical signal monitor, the optical signal switching unit and a control part for changing the states of optical switches when the optical signal monitor detects disconnection of an optical signal for automatically restoring the transmission line to the normal state.
As shown in FIG. 8, optical switches 111 employed in a conventional optical signal switching unit are provided for optical waveguides 105 of polymer films 102 formed by waveguide storage sheets pressed by keep plates 103. As shown in FIG. 9, the waveguide storage sheets 102 bring notch end surfaces 102a into close contact with each other on notches in a passing state (hereinafter referred to as an ON state) implemented by not pressing a driving mechanism (push rod) 106 provided on an intersection between the waveguides 105. Therefore, cores forming the waveguides 105 are also in close contact with each other on the notches. Thus, light reaching the notches brought into close contact with each other passes through the notches as such.
In a course changing state or a reflecting state (hereinafter referred to as OFF state) implemented by pressing the push rod 106, the push rod 106 pushes the notch end surfaces 102a upward and separates the same from each other for bringing the notch end surfaces 102a into contact with the air, as shown in FIG. 10. Therefore, light propagated through the waveguides 105 having a high refractive index cannot exit from the notch end surfaces 102a but is totally reflected by an air space in a clearance 109 and propagated while changing the traveling direction by about 90xc2x0.
According to the aforementioned mechanism, input light components L1 and L2 continuously pass through portions where the notch end surfaces 102a are in close contact with each other on the intersections between the waveguides 105 when optical connection is in an ON state on the intersections between the waveguides 105, as shown in FIG. 8. When the optical connection is in an OFF state in the intersections between the waveguides 105, on the other hand, both of the input light components L1 and L2 are totally reflected by the optical switches 111 and output in a prescribed direction while changing the traveling direction by about 90xc2x0. As hereinabove described, the feature of the conventional optical switches 111 resides in that the destination of the light is switched by pressing/pulling the single push rod 106.
However, all of the aforementioned optical switches 111 can take only binary states, i.e., the ON and OFF states. When the states of the optical switches 111 are changed as described above, therefore, the optical signal is instantaneously cut off following this change. In this case, the optical signal monitor determines that the optical signal is disconnected. In other words, the optical signal monitor falsely recognizes the optical signal as disconnected when the states of the optical switches 111 are simply changed, due to the instantaneous cutoff of the optical signal following the operation of changing the states of the optical switches 111.
When a light amplifier is arranged at the bark of the optical switches 111, such instantaneous cutoff of the optical signal caused by the optical switches 111 may cause an optical surge in the light amplifier and damage another optical element, such as a photoreceptor. In a proposed countermeasure, therefore, a constant latency time is provided between a time detecting cutoff of an optical signal and a time switching the optical signal (Japanese Patent Laying-Open No. 2000-13315).
When such a latency time is provided, however, network switching is undesirably delayed.
An object of the present invention is to provide an optical signal switching unit causing no instantaneous cutoff in switching without providing a latency time.
The optical signal switching unit according to the present invention has a plurality of input waveguides and a plurality of output waveguides for switching courses of each optical signal input in the input waveguides on intersections between the input waveguides and the output waveguides and outputting the optical signal to the output waveguides. This optical signal switching unit comprises a plurality of optical switches arranged on the intersections for switching courses of the optical signal and a switch state control part controlling the states of the plurality of optical switches thereby controlling formation of the output optical signal. The optical signals take binary states including a first course state providing the optical signal with a first course and a second course state providing the optical signal with a second course different from the first course as well as an intermediate state partially including both of the binary states.
According to this structure, an output from any output waveguide connected to an optical signal monitor, for example, can be prevented from reaching zero without providing a latency time or the like when forming an output optical signal from an input optical signal by the optical switches. When all optical switches are in the intermediate state, for example, the output from the output waveguide does not reach zero. When all optical switches are set to either one of the said prescribed binary states while maintaining the intermediate state and approaching the state of each optical switch to a prescribed one of the binary states, switching can be performed without reducing the output to zero. Therefore, the optical signal monitor makes no false recognition of disconnection or the like, and no optical surge is caused in an optical amplifier or the like provided on an output side.
When the aforementioned two types of waveguides are linear waveguides, for example, the aforementioned first course state of the optical switches is a passing state passing the optical signal as such, and the second course state corresponds to a course changing state changing the course of the optical signal to a direction intersecting with the direction of propagation thereof. The aforementioned switch state control part may have a structure capable of continuously controlling not only the aforementioned binary states but also the ratio of the binary states in the intermediate state, or a structure capable of discretely selecting and controlling only a specific ratio, as a matter of course.
The switch state control part can control the states of the plurality of optical switches to cause no zero signal in an output waveguide connected to an optical signal monitor monitoring the output signal.
According to this structure, the optical signal monitor makes no false recognition of disconnection, and no optical surge is caused in a light amplifier provided in a rear stage.
All output waveguides provided on the optical signal switching unit or a single arbitrary output waveguide may be connected to the optical signal monitor.
The switch state control part can vary an inter-state transfer rate for changing the states of optical switches with every optical switch.
According to this structure, the period of the intermediate state of each optical switch can be arbitrarily adjusted while shifting the times for starting changing the states of the respective optical switches. Consequently, the magnitude of fall of an output value can be readily controlled when switching a specific output waveguide.
The inter-state transfer rate can be increased along the order of changing the states of the plurality of optical switches.
Also according to this structure, a redundancy time for the intermediate state between the optical switches can be increased for further reducing fall of an output value following switching in the output waveguide. Further, an output to the output waveguide can be readily prevented from reaching zero.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.